A summary of an idea from Goldratt's Critical ChainWhen talking about task duration estimates, I'll define a "true" estimate as an estimate in which we have 50% confidence, that is, an estimate that has a 50% probability of being greater than or equal to the actual duration. Similarly, a "safe" estimate is an estimate in which we have 90% confidence.
The area under the curve from 0 to 1 is about 0.5 (as you can see noting that the green area of the graph is slightly less than a 1 x 0.6 rectangle), so the probability that this task will take less than 1 week is about 50%. In other words, the 50th percentile time for this task is 1 week. The probability that it will take between 1 and 2 weeks is about 25%. The probability that it will take between 2 and 4 weeks is about 15%. And the probability that it will take longer than 4 weeks is about 10%. Adding these numbers up, we see that this task has a 50% chance of finishing in less than a week, a 75% chance of taking less than 2 weeks, and a 90% chance of taking less than 4 weeks.
So, an estimate with 90% confidence—a safe estimate—will have to be many times longer than a true estimate.